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51 using a distance measuring instrument (DMI) mounted to the vehicle wheel or to an external distance wheel. Equip- ment specifications are listed in Table B.1. Data Output and Display The field operation and playback software should be capable of the following displays: ⢠Direct time domain waveforms from each of the two receiv- ers (Figure B.2); ⢠Dispersion curve for each wheel pair (Figure B.3); ⢠Waterfall plot of dispersion curves collected versus dis- tance covered for each wheel pair; and ⢠Output format should be a volume of data with surface wave velocity as a function of x (longitudinal distance), y (transverse offset), and z (depth). Data Collection Protocol Due to the relatively low collection speed, this system should be used exclusively for project-level work, with a focus on spe- cific locations and areas of concern. The start and end points of data collection should be referenced with mile markers or other fixed reference points. Landmarks encountered during the survey (e.g., intersections, bridge decks) should be anno- tated with some type of manual marker in the data. The data should be observed visually during collection to ensure that the system is operating properly and that the expected features are appearing in the data. Data Analysis The objective of the analysis is to identify locations where a sharp drop in surface velocity with depth is associated with delamination (debonding) of an asphalt layer from the layer Basic Principle In the spectral analysis of surface waves (SASW) test, the pavement is struck with a short, high-frequency source, thus creating a surface wave that propagates away from the source. Two receivers spaced near the source, but at dif- ferent distances, detect the arriving surface wave, and data from these two locations are used to calculate the curve of wavelength versus frequency (dispersion curve) for the sur- face wave. Because wavelength is related to depth of pene- tration, this dispersion curve is interpreted as a relationship between surface wave velocity and depth. A sharp drop in velocity at a particular depth is indicative of a discontinu- ity in the pavement structure, which would be associated with delamination (debonding) and stripping. Automated equipment has been developed to carry out this test con- tinuously at a slow walking speed. Software to analyze the data is partially automated but still requires considerable user interaction. Equipment and Operation The SASW system consists of an array of rotating sensor wheel pairs, with each pair of wheels spaced approximately 2 ft laterally from the adjacent pair. The purpose of the array is to collect equally spaced parallel lines of data simultane- ously so that coherent areas of delamination can be iden- tified and mapped. Figure B.1 shows an example of such equipment. In that example, each wheel is approximately 1 ft in diameter and is mounted with six displacement sensors and impactor pairs. Each sensor/impactor pair is spaced at 6 in. intervals around the circumference of the wheel. Each sensor wheel pair is coupled with a rubber iso- lated axle. Data are collected continuously at 6-in. intervals while the system rolls along the surface of the pavement. The data collection for each wheel is independently trig- gered, and the position of the collected data is obtained by Technical Brief: Spectral Analysis of Surface Waves A P P E n D i x B
52 below. To accomplish this task, the surface wave profiles at each location (Figure B.3) are assembled as a full volume of data, where x and y are the coordinates of the test point, z is the wavelength (or depth), and the value at point (x, y, z) is the surface wave velocity. Surface wave velocities can be pre- sented for each selected depth range (or âdepth sliceâ), and the velocity values within the slice are presented in grayscale. In the example in Figure B.4, velocities range from 3,000 ft/s (light gray) to 5,000 ft/s (black). The higher the surface wave velocity, the better the condition of the pavement. Anomalies can be seen as light spots where the velocities are lower. It is clear in this example that there is a sharp drop in velocity at a depth of approximately 0.4 ft, suggesting a delamination at that depth. Analysis of the depth slices shown in Figure B.4 can be labor-intensive. It would be desirable to have data analysis software that can automatically search through the volume of data, detect locations where there is a sharp drop in the velocity with depth, and report those locations and depths in a plan area map that can be readily interpreted by a highway engineer. (a) (b) Sensor Impactor Distance wheel Transducer Wheel Pairs Data Acquisition Computer Figure B.1. IE/SASW scanning system (test setup of 6-in. transducer spacing and 2 ft between pairs): (a) layout of sensor wheel array and (b) detail of sensor wheel pair. Table B.1. SASW Equipment Specifications System Type Array of Pairs of Rotating Sensor Wheels, Lined Up Transverse to the Direction of Travel Sensor frequency response Up to 50,000 Hz Impact source input frequency Up to 50,000 Hz Lateral spacing between wheel pairs Variable (2.0 ft typical) Lateral coverage per pass 6 ft with three wheel pairs (half-lane width) Longitudinal data collection rate 1 test per ft (minimum) Travel speed during data collection 1 to 2 mph Travel speed during mobilization Posted speed limit Real time display Single wheel pair waveforms at reduced display rate System monitoring and control Within or outside survey vehicle Data collection rate Based on speed and sensor spacing on sensor wheel Spatial reference Vehicle DMI or external distance wheel Source A m pl itu de First Sensor Second Sensor Figure B.2. Detected waveforms.
53 Equipment Availability and Cost The SASW continuous scanning system discussed in this brief is not available commercially. A prototype system has been developed and tested by Olson Engineering. Also, the data interpretation and use of the analysis software in its cur- rent state require a fair amount of experience. An estimated cost of a fully operational system is not available. Advantages and Limitations The primary advantage of an SASW system is its ability to detect delamination (debonding) within asphalt pavement layers directly. The principal limitation is speed. The system is restricted to testing at walking speed and, thus, its appli- cation is limited to project-level analysis and diagnostic investigations. (a) (b) Su rf ac e w av e Su rf ac e w av e Figure B.3. Dispersion curves calculated from SASW data: (a) dispersion curve from sound pavement and (b) dispersion curve from pavement with debonding at depth of 5 in.
54 Surface Wave Velocity (ft/sec.) Depth = 0.1 â 0.2 ft Depth = 0.2 â 0.3 ft Depth = 0.3 â 0.4 ft Depth = 0.4 â 0.5 ft Depth = 0.5 â 0.6 ft Depth = 0.6 â 0.7 ft 3000 3100 3200 3300 3400 3500 3600 3700 3800 3900 4000 4100 4200 4300 4400 4500 4600 4700 4800 4900 5000 Figure B.4. Depth slices of SASW velocity versus depth data.
